This invention relates generally to a precision waveform signal source apparatus, and in particular to a triangle waveform generator having a network to compensate for current-switching control loop delays.
Conventional triangle waveform generators operate on the principle of charging and discharging a capacitor at constant rates to generate extremely linear up- and down-ramp voltages. Typically, triangle waveform generators include a current-switching control loop in which a comparator senses when the capacitor voltage has ramped to a predetermined limit and switches, reversing the direction of capacitor current so that the voltage begins to ramp in the other direction. However, there is a finite amount of delay time between the point at which the ramp voltage reaches the comparator reference level and the point at which the ramp voltage reverses direction because of the propagation times of the switching devices and the delay caused by the hysteresis effects of the comparator. The effect of this loop-delay behavior is the generation of triangles that become larger and larger as the frequency is increased.
The conventional solution to the foregoing problem has been to insert an RC peaking network into the loop between a buffer amplifier connected to the capacitor and the comparator input to boost the amplitude of the higher-frequency triangles, thereby causing an early switching of the comparator to compensate for the total loop delay. This is not a complete solution to the problem, however, because of the initial condition and charge rate of the compensation capacitor. That is, the conventional technique provides an approximate solution to the loop delay problem for free-running balanced-symmetry triangles, but not for triggered, gated, burst, or variable-symmetry operation.